The invention relates to an automatic frequency offset compensation method to compensate the frequency offset between transceiver equipments, and to a device to implement such a method.
The technical area is radio communication at specific ranges of frequencies, in particular for applications that need competitive solutions in terms of capacity to control the channels and of low battery consumption. In most of the radio frequency modules, by now, the frequency depends always on the accuracy of a frequency reference (crystal, resonator, . . . ).
A PLL (Phase-Locked loop) with a crystal oscillator is generally used. But the accuracy of this module is linked to the accuracy of the crystal. The accuracy depends on three parameters: the batch (initial dispersal at room temperature), temperature and aging. The dispersal is easily cancelled by calibrating the module in production. For the second parameter, temperature compensation can be used,—by using a compensation curve based on a average of the frequency offset for different batch—, but it remains an error, especially at the extreme temperature, which can not be removed before reception.
The effect of the third parameter (aging) cannot be neutralised. So the consequence is that there is always a frequency offset between the transmitter and the receiver. If the frequency offset is too large, the communication becomes impossible. For narrow band device, the allowed frequency offset is low, due to the narrow bandwidth. The narrow bandwidth has the advantage to allow best sensitivity. However, considering a communication device as a whole, a compromise should be found between good sensitivity and large allowed offset width.
One solution should be to implement several receptions at different adjacent sub-channels, over a ranger larger than the receiver bandwidth. This allows covering a wide range even with a narrow bandwidth. But, several receptions increase the consumption for scanning a channel, especially in case of empty channels. The time period for scanning is also increased.
Another solution is the reduction of the frequency offset by using more accurate frequency reference as TCXO (temperature crystal oscillator). The drawbacks of this solution are higher costs and consumption. Thus, the problem is the achievement of a communication between a receiver and a transmitter even with a frequency offset larger than that the receiver can allow, without time loss and extra consumption.
The invention comes from searching a solution to reduce the consumption while keeping a standard crystal and from the observation that the frequency offset could be deduced from demodulation. And, if the frequency offset is known, then compensation can be obtained by appropriate looping of optimised detection. More specifically, the object of the invention is an automatic frequency offset compensation method to compensate the frequency offset of a carrier frequency signal modulated by data between emitter and receiver equipments, characterised in that the method consists in, after signal digitalization and next channel filtering carrying out a demodulation to obtain the I (In Phase) and Q (Quadrature) parameters, calculating the frequency offset from I and Q parameters between received and current frequency of the equipment, and synthesizing corrected frequency based on the calculated offset.
The automatic frequency offset compensation method to compensate the frequency offset of data modulated by carrier frequencies between emitter and receiver equipments. The method consists in, after frequency digitalization and next channel filtering carrying out a frequency demodulation to obtain the I (In Phase) and Q (Quadrature) parameters, calculating the frequency offset from I and Q parameters between received and current frequency of the equipment, and synthesizing corrected frequency based on the calculated offset. The I,Q demodulator allows the offset frequency deduction between the received signal and the current frequency of the receiver. The digitalized frequency is different from the current frequency because of crystal dispersal. The method allows a reduction of time for scanning and so a reduction of the consumption. It allows the use of a standard crystal as frequency reference for the receiver and the transmitter.
With preferred embodiments:
the filtering, comparison and frequency offset calculation loop can operate once but is advantageously repeated at least twice for a better efficiency;
the frequency compensation is optimized by using a coarse detection with a wide filtering, advantageously repeated, for the reception of a preamble signal, the first part of the data, and a fine detection with a narrow filtering for reception of the useful data frame, the remaining part of the data; this effect results from the fact that a frequency coarse detection with a wide filter gives the same reception performances as a frequency fine detection with a narrow filter;
a synchronous mode is used for the fine detection;
an asynchronous mode is used for the coarse detection.
The object of the invention is also a device for automatic offset frequency compensation of the frequency received by a transceiver, comprising the following components:
a frequency receiver,
a filter,
an analog to digital converter,
a channel filter
an I/Q demodulator,
an automatic frequency control (AFC) unit for calculating the resulting offset correction;
a frequency synthesizer to provide the corrected frequency,
a digital unit to control the components.
A transceiver encompasses the equipments that transmit and/or receive data carried on appropriate frequencies in the radio-frequency range.
According to preferred embodiments:
the frequency synthesizer and loop arrangement are provided by a unique Phase Locked Loop (PLL) synthesizer;
the channel filter is a programmable-channel filter to improve the result of the AFC unit;
a register stores the output information of the I/Q demodulator to be provided to the AFC unit;
a digital unit such as a microcontroller controls the AFC unit, the channel filter, the demodulator, and the PLL synthesizer;
an analog to digital conversion of the filtered frequency is achieved by a Sigma Delta ADC Converter;
a bit synchronizer is implemented for the synchronous mode;
the microcontroller is a 8 bit microcontroller.